I have a small part of a big book where maybe the cover wants to peel off 50 pages or so. It would be fine if I could Reglue it back to those pages. What is the best glue to make it stick? Elmers definitely didn't do it 

Library glue or whatever Libraries use.  You might have to order it online.  Google it and/or ask a library what they use, etc.

I've repaired a few Hardbacks that were begining to come apart - I had tried using Elmers Glue, but found that the Gorilla Glue worked just fine - I used a couple of toohpicks to apply the glue - using a W (or M) shape when glueing the cover back on and just apply a very small amount if a section (seam) was begining to come apart. You only need a very little bit - after it's applied just set it aside with a small weight laying on top (I alsways put another book on top after proping the repaired part upright) - leave things alone for a day and checkout your handi-work the next day.

Good luck Jamie...

I like Aleene's Tacky Glue. 

Gorilla glue expands when used so use it sparingly.  I like Aleen's tacky glue myself.  I don't recall what we used in the library.  Some cool stuff in like a little paint can.  We used very slender wooden dowels to work it into the crevices of the book and down the hinges of a hardcover to firm them up.